1. Field of the Invention
The present invention relates to a semiconductor device, more specifically to an etching apparatus suitable for controlling the concentration of an etching solution, a method of controlling the concentration of the etching solution, and a method of manufacturing a semiconductor device using the etching apparatus.
2. Description of the Related Art
A semiconductor manufacturing process using an etching solution containing sulfuric acid (H2SO4) and hydrogen fluoride (HF) as main components has been proposed as a process of etching of films having differing qualities on the surface of a semiconductor substrate. In the process, it is important to maintain the etching solution at low water content, and an initial water concentration is regulated to be at 5 wt % or lower. However, when a treatment chamber is used for a batch treatment method, it is basically open to the atmosphere. Consequently, the concentration of sulfuric acid in the solution is gradually reduced due to moisture absorption by sulfuric acid from the ambient atmosphere. Moisture absorption by sulfuric acid changes the state of dissociation equilibrium of an etchant contained in the solution, and the etching rates and selective etching ratio of intended etched films thus deviate from the initial states thereof by a large extent.
In order to prevent moisture absorption of sulfuric acid, there is a method of purging the ambient atmosphere of the treatment chamber with a bulk gas such as nitrogen (N2). However, it is almost impossible to create a fully sealed state after a certain level, by sulfuric acid cannot be completely suppressed. Hence, etching rates change. Consequently, in order to remove the intended etched films selectively, treatment time for the etched films should be adjusted, or process control should be examined with changes in etching rates taken into consideration in advance. Thereupon, there is a method, which has been examined, to control the concentration of sulfuric acid by means of suppressing an increase of water due to moisture absorption by sulfuric acid. This method is done by spiking highly-concentrated sulfuric acid when the concentration of sulfuric acid is decreased.
However, as for the semiconductor manufacturing process using an etching solution containing sulfuric acid and HF as main components, not only sulfuric acid but also HF, as an etchant, gradually evaporate. Hence, it is difficult to control the etching characteristics of the etching solution so that the etching characteristics are invariable.
Currently, apparatuses capable of controlling the concentration of sulfuric acid in the solution include a sulfuric acid-hydrogen peroxide aqueous solution monitor (SPM), an ammonia-hydrogen peroxide aqueous solution monitor (SC1), and a hydrochloric acid-hydrogen peroxide aqueous solution monitor (SC2). However, the SPM is used for the purpose of keeping the concentration of hydrogen peroxide constant. Thus, it is difficult to keep the concentrations of sulfuric acid and water under constant conditions. In other words, the SPM is a concentration controlling apparatus for refilling new hydrogen peroxide when the hydrogen peroxide in the solution is reduced by being dissolved in water. Thus, the water content in the solution increases as hydrogen peroxide is refilled. Accordingly, the water content cannot be kept at 5 wt %, and the concentration of sulfuric acid also deviates. If an attempt is made to maintain the concentration of sulfuric acid at a certain level by using the SPM, a large quantity of sulfuric acid should be added to the solution in order to suppress an increase of water due to dissolution of hydrogen peroxide into water, and thus costs for chemical liquids are increased.
Further, as for the SC1 and the SC2, quantities of hydrochloric acid or ammonia should be added to the solution in order to attenuate the dissolution of hydrogen peroxide into water. Thus, quantities of the chemical liquids should be refilled for maintaining the concentrations of both water and sulfuric acid at a certain level.